I've had Rachael as my main amp for a year-and-a-half now, and thought it was a good time to start tube rolling. I'm doing a "slow roll" of the entire set (input first, then rectifier, then output last) letting each stage burn in for about a week until moving to the next.

So, I swapped out the 6N1P's last week for a new set of 6N2P's last week. All good. Love the slight boost in gain, a bit more controlled and clear all around.

I then moved to rectifier about 2 hours ago. For this I decided to swap the stock 5U4 for a Mullard GZ34 (equivalent of a 5AR4). And this is where the issue comes up. The amp sounds just fine, no clipping, and actually no real noticeable difference from the 5U4 yet, but still hasn't burned in obviously. The problem is that the bias readings are way higher. Before the meters would read between 44 and 48 and they were equal. With the GZ34 the left has jumped to 52, and the right has jumped to 56.

Is this a problem? Can it hurt the equipment? As I said, I haven't noticed any clipping or distortion even at very high levels. In fact the sound is evermore glorious. FWIW, the speakers are Klipsch Cornwalls, quite sensitive, so very difficult to drive over the edge. The new GZ34 is new production, not NOS. The current EL34's are still stock, and were set to be replaced by new production Mullard EL34's some time next week.

Out of caution, I would change the rectifier tube back until you can get good info on the new tube. The bias readings are what concern me most, since they are higher and not equal. if the meters are showing a higher reading for the input tubes, then it may be possible that the output tubes are also in the same position.

Imageflood- I would echo 4krow's comments and concerns. The bias seems pretty high with that rectifier, I've never had numbers up that high. Keep us posted on what you find out. Happy listening, Chris.

Thanks for your thoughts. I still have the GZ34 in for the time being, and I'm cautiously optimistic... I double-checked to make sure all the tubes were properly seated. I don't know if that did the trick, but both sides settled at 52. Still too high I suppose, but then again the GZ34 should have less sag than a 5U4 and should yield higher bias readings.. I also took the liberty of swapping in the new output tubes (Mullard EL34), and getting the exact same result, 52/52.

So now I suppose I am not so worried about the amp itself catching fire (!!). I am concerned about tube life, but I want to let it burn in a bit to watch it. A secondary issue may be that there are simply too many Mullards in the system. They're sounding overly warm so far, so I may switch back to the 5U4 simply for more clarity.

After a few days I've had to swap the stock 5U4 back in. Too many new tubes at once. Things weren't sounding totally easy, but yesterday couldn't help but hear distortion/clipping in the upper range. Listening to Brazilian music so could have been reco-reco or some other percussion instrument, but something didn't feel kosher. The preamp tubes are pretty well burned in, so couldn't tell if the source of the problem is the EL34 or the GZ34. But since it continued to "burn hot" (bias btw 52-56) I took the advice above and cooled back down with the 5U4 for the meantime. Bias resting comfortably at 48 now. It's not sounding very exciting yet, but best to let the power tubes burn in and open up before doing anything else. Will update when I make the next move.

FWIW, I'm not yet sold on new production Mullards. Maybe my ear has become more analytical, but I need more clarity and transparency from signal to speaker. Again, still have a lot of burn in left on them before I reach a final conclusion. Any recs for alternative EL34's much appreciated though.

A quick call to Steve would very likely provide insights into any bias function and sound related to bias questions. I wonder if those of us without meters experience this without knowing? I would be surprised if 5AR4/GZ34 were not acceptable for the Rachel, but who knows, maybe you have a hot one. I ended up using JJ GZ34 in my SE34 (pre-Rachel) and really liking the GZ34/5AR4 dynamic character, definition and space. This was quite a few years back, but if I recall correctly, the JJs were pretty open and clear tubes for the type, fitting my preferences for open neutrality.

With my Toriis though (and maybe I would feel this way with a Rachel now?), GZ34s have been too much for my preferences and system/rooms. They work great for many, but for me they just push the other tubes a little too hard...not necessarily into distortion, but too much signal density, causing an overblown sound and reduced open spaciousness. The focussed density of instruments in space can easily fill the sense of space between instruments. There is a place where it is all "right," great tonal density with great ambient space, the right balance allowing greater fine detail also.

This is what led me to Phillip made GZ32s, having the GZ definition, space and speed, with less signal density than GZ34s...a toned down version of the type. But they are usually relatively expensive. Some 5V4Gs (American designation for GZ32) can be quite similar in balance and character, though having their own varied personalities too depending on make and vintage. Especially the earlier fat bottles with D getters can be really good tubes and inexpensive alternative to GZ32s. Like this: http://www.ebay.com/itm/True-NOS-NIB-Sylvania-USA-5V4GA-Get-Black-Plate-Vacuum-T...

The ones I have that look like this are a little less refined in terms of very fine detail, texture and complexity than Mullard/Phillips type GZ32s. But here they have quite a nice balance top to bottom, being more about openness and fine detail with a bit more liquidity than the fine detail with texture and openness of the GZ32...also a nice sound. My pair of these Sylvanias also goes a little deeper and is more dynamic and focussed than the Mazda labelled Phillips GZ32 I have been using, also nice in its own way. The focus and open clarity of the Sylvania can be a little too intense depending on other tubes and everything else, especially if your room/system tends to hot/saturated notes....like strong upper range piano hits or similar trumpet notes. Right now they sound pretty great here though...

6N1Ps are prone to earlier distortion in my Decware experience, and to a lesser degree, ECC88/6DJ8s can often distort easier than 6922/E88CC. I can't really remember how 62NPs fit into this, but it may be they are being pushed a little hard by the GZ34 bias change. Again, Steve could clarify this.

I ran some cryo'd reissue Mullard EL34s for quite a while in my MKIII. It has been a few years ago, but I remember they were in the "long burnin category" compared to many power tubes, and were particularly "not good" until they burned in more. I got mine from a forum member who gave up on them as unbalanced and unresolved with maybe 100 hours on them. When I put them in here, I heard the same burnin things I had heard before with slow burnin power tubes...and hoping this was the problem, kept burning them in.

In burnin, the highs tend to be harder, detail more consolidated/less complex, and the mids down, thickish, darkish, undefined, also lacking complexity. This makes an uncomfortable balance issue (for some tubes more than others) beyond lack of refinement....murkier low mids down contrasting too much with a sort of crudely defined/more articulate and open top. With these tubes, this imbalance was pretty bad I think...the upper ranges a little sizzley and the bottom sluggish and thick.

I think it was over 150 hours before I started to hear them. Then they got better for a while, the bottom more defined and clearer, and the detail more complex and natural...relative balance finally. Here, in time, I got to like them a fair bit at the time.

With each of our system/rooms so different, it is hard to guess the best synergistic combination of tubes, but hopefully these experiences will somehow help you find yours.

Thanks for the way more than comprehensive reply! It did cross my mind that the 62NP's might be a culprit in this. But at this point I think it's best to let the EL34s burn in fully and see where we're at. Man, I had no idea it would take so long, though, to burn in those Mullards. It will like be September before I can log 150 hrs on them, and by that point one gets a sense of diminishing returns... What you described at either end of the spectrum was spot-on when they first went in: flabby bass and, worse, crackling highs, almost like bacon sizzling at times. I may have mistaken that for clipping. I must have forgotten how painful burn-in can be!

In any case, the power tubes are performing better since I went back to the 5U4. And probably have to leave things be for the rest of summer (!!) so that I can best determine how to hit the best synergy. Maybe the Mullard GZ34 will be friendlier at that point, or look for another brand or go to GZ32 as you mentioned. In the preamp stage, I've found 7DJ8 much to my liking in other units, so will pick up a pair of those and have a full comparison.

Thanks again for all the help. I'll check in with Steve for more answers.

I think those Mullard EL34 copies were the worst tubes I have dealt with in terms of burnin. Seems most power tubes sound OK new, some even pretty good, but power tubes are the worst for burnin blues in my experience....same basic sound issues, murky bottom and rigid top, but most are easier to wade through burnin than the Mullards.

I have been using 807s for a while now, and they are the best for burnin of all I have tried. This is part because they are often a little used, but I have some Amperex Holland made, and some early RCAs, both clearly NOS, and both were quite listenable new...better with some time, but nice!

Sounds like a good plan to ride out the Mullard burnin before making big moves, that is if you can hack it. I guess your amp came with Tungsol EL34B? What did you think of those?

If you are using a Shuguang 5U4G, might be good to keep an eye out for a nice scoring RCA 5U4G-ST. They come up quite inexpensively at times. Especially earlier 50s RCA made 5U4G STs are very musical and refined, but I like some from the sixties pretty well too. I tend to prefer those with top/side getters for a more open sound with more complex fine detail. Some of the bottom getter ones, with heavy plate structure, are good tubes, but bassy/darkish for me.

Well, by this point the edges have been taken off the Mullards, so that's good. Now they're merely unexciting, but at least not harsh. Still going to keep at them, though, to see if they open up. I have been really interested in 807's too -- I think there's a pic of them in the SE34I.5 on the Decware page, and I've heard good things about them on the forums. I'm likely to pick up a pair of NOS RCA's soon, and eventually rotate them in. In retrospect, I'm very much impressed by the Tung Sol's and the stock tubes in general. Very neutral (in a good way), don't add or subtract, just do their thing and get out of the way. After almost 2 years I needed a change, but I'll always have a pair of the Tung-Sol's on hand just to pop in once in a while.

For the moment I'm not playing around with rectifiers until the power tubes settle in. Steve got back to me and basically the GZ34's don't pose any immanent risk to the amp itself, but will shorten tube life a bit. Rachael makes so few demands on tubes, though, that it may not be a bad trade-off. But that I'll make that determination a few months from now.